Star polymers useful as viscosity index improvers for oils and lubricants are described in U.S. patent pending application Ser. No. 942,019 filed Sep. 8, 1992 (T4773N). The star polymers have blocks of a methacrylate at the end of polymeric arms that are grown from a coupled core of a smaller star polymer. The smaller star polymers are prepared by crosslinking small blocks of styrene, isoprene, or butadiene with divinyl benzene. The larger polymeric arms are grown at active lithium sites on the central core of the small star polymers. The methacrylate blocks at the end of the polymeric arms have a strong tendency to couple two or more of the star polymer cores. Coupling of the star polymer cores effectively broadens the molecular weight distribution of the star polymers and reduces performance as viscosity index improvers for lubricating oils. Elimination of the coupling would improve performance in lubricating oils.
Addition of amide or imide groups to polymeric viscosity index improvers increases the dispersency of sludges in lubricating oils. Conversion of acrylic groups in acrylic polymers to amide or imide groups is described in U.S. Pat. No. 4,246,374 which teaches reaction of the acrylic group with an anhydrous primary amine. The conversion reaction occurs between outer limits of about 200.degree. C. and 450.degree. C.
Conversion of the acrylic groups to amide groups will occur in the presence of other monomer units such as styrene, butadiene, or isoprene. However, butadiene and isoprene units in the polymers thermally degrade at temperatures between about 200.degree. C. and 450.degree. C. Conversion of acrylic groups to imide groups at lower temperatures would reduce degradation of polymers containing butadiene or isoprene.